A metal strip such as a steel strip, generally manufactured by rolling a raw material using mill rolls, may have a defective flatness caused by a non-uniformity in thickness in the width direction of the strip. More particularly, types of defective flatness include center buckling, quarter buckling and wavy edge. Such a defective flatness of a metal strip is produced during such processes as continuous hot rolling, skin-pass rolling and continuous cold rolling. Accurate and quick measurement of a defective flatness of a metal strip, if possible, would give the following effects:
One of the known factors quantitatively representing the degree of defective flatness of a metal strip as mentioned above is the steepness. When the wave length at the portion of the metal strip having the above-mentioned defective flatness is l.sub.1, and the amplitude is h.sub.1, then the steepness is expressed as h.sub.1 /l.sub.1.
Since, when a defective flatness occurs in a metal strip during rolling, a variation is produced in the relative distance (hereinafter referred to as a "vibration"), having an amplitude and a frequency corresponding to said defective flatness, between said metal strip and a distance-detecting means arranged adjacent to the surface of said metal strip, a method for continuously measuring the steepness of a defective flatness of a metal strip during rolling has been proposed, which comprises:
As a distance-detecting apparatus to be used in the above-mentioned step (a), i.e., the step of detecting a vibration produced in a metal strip during rolling, for example, the following three distance-detecting apparatuses are disclosed in Japanese Patent Provisional Publication No. 53,843/76 dated May 12, 1976:
The above-described distance-detecting apparatus (I) operates as follows. In a balanced state of the AC bridge, when the metal strip during rolling vibrates under the effect of a defective flatness thereof, there occurs a change in the impedance of the distance-detecting coil. As a result, an output appears at the output terminal of the AC bridge, which corresponds to a vibration in the relative distance between the metal strip and the distance-detecting coil. This output appears at the output terminal of the differential amplifier as the output signal of said vibration in relative distance.
The distance-detecting apparatus (II) operates as follows. In a state where the resonance frequency of the resonance circuit of the parallel resonance element becomes equal to the oscillation frequency of the oscillator through adjustment of the capacity of the capacitor, when the metal strip during rolling vibrates under the effect of a defective flatness, there occurs a change in the impedance of the distance-detecting coil. As a result, the resonance frequency of the resonance circuit shows a change corresponding to the variation in the relative distance between the metal strip and the distance-detecting coil, thus causing an output signal of said variation in relative distance to appear at the output terminal of the positive feedback amplifier. The distance-detecting apparatus (II) is characterized in that it is possible to control the quality factor of the resonance circuit by adjusting the degree of amplification and/or the amount of positive feedback of the positive feedback amplifier, thus enabling to linearize the output characteristics of the output signal of said variation in relative distance. The above-mentioned distance-detecting apparatus (II) is disclosed also in Japanese Patent Provisional Publication No. 80,860/75 dated July 1, 1975 and Japanese Patent Provisional Publication No. 51,963/76 dated May 7, 1976.
The distance-detecting apparatus (III) operates as follows. When the metal strip during rolling vibrates under the effect of a defective flatness, there occurs a change in the impedance of the distance-detecting coil through which an AC current of the oscillation frequency of the oscillator flows. As a result, the differential amplifier shows a change in the degree of amplification corresponding to the vibration in the relative distance between the metal strip and the distance-detecting coil, thus causing an output signal of said variation in relative distance to appear at the output terminal of the differential amplifier. The distance-detecting apparatus (III) is characterized in that, while the changing characteristics of the impedance of the distance-detecting coil corresponding to said variation in relative distance are non-linear, the changing characteristics of the impedance of the distance-detecting coil are compensated by adjusting the degree of amplification and/or amount of positive feedback of the differential amplifier, thus resulting in linearization of the changing characteristics of the output signal from the differential amplifier corresponding to said variation in relative distance.
A method for measuring the steepness of a defective flatness of a metal strip during rolling is disclosed in the above-mentioned Japanese Patent Provisional Publication No. 80,860/75, which comprises arranging a scanning mechanism at a prescribed position below a metal strip during rolling and adjacent to the surface of said metal strip, and holding a distance-detecting coil movably on said scanning mechanism, thereby detecting a vibration of said metal strip at any position on said metal strip.
As a means to be used in the above-mentioned step (b), i.e., the step of detecting the amplitude and the frequency of a vibration produced in the metal strip, the conventionally known means include a detector circuit for detecting an output signal from the distance-detecting apparatus to obtain a voltage corresponding to the amplitude of said output signal as a voltage corresponding to the amplitude of the vibration produced in said metal strip, and a frequency/voltage converter (hereinafter referred to as an "F/V converter") for obtaining a voltage corresponding to the frequency of an output signal from the distance-detecting apparatus as a voltage corresponding to the frequency of the vibration produced in said metal strip.
As a means to be used in the above-mentioned step (c), i.e., the step of calculating the steepness of a defective flatness of a metal strip from the detected values of amplitude and frequency of a vibration produced in said metal strip, a calculating circuit is conventionally known for entering an output signal from a detector circuit, an output signal from an F/V converter, and an output signal of the travelling speed of the metal strip (for example, an output signal having a voltage corresponding to the travelling speed of the metal strip is obtained through a tachometer connected to the rotation shaft of a support roll or a deflector roll for the metal strip during rolling), and calculating the steepness of a defective flatness of the metal strip by the following formula from the voltage values of these output signals: ##EQU1## where, .sigma.: steepness,
However, the conventional method for continuously measuring the steepness of a defective flatness of a metal strip during rolling, which comprises the above-mentioned steps (a), (b) and (c), has the following problems. More specifically, FIG. 1 is a descriptive drawing illustrating an example of vibration of a metal strip during the travel thereof between two support rolls arranged at a prescribed distance. In FIGS. 1, 4 and 5 are support rolls, and 1 is a metal strip travelling between the two support rolls 4 and 5. A vibration having an amplitude h.sub.3 and a wave length l.sub.2 corresponding to a defective flatness, as shown by a solid line, is produced in the metal strip 1. In addition, irrespective of the presence of a defective flatness, a natural vibration "a", as shown by a chain line, is necessarily produced in the metal strip 1 during rolling. Thus, the vibration corresponding to the defective flatness and the natural vibration are detected in the form of a composite vibration by the distance-detecting apparatus. Therefore, since the value detected by the detector circuit and the value detected by the F/V converter are affected by the natural vibration, it is impossible to accurately calculate the steepness of the defective flatness of the metal strip 1 during rolling through the calculating circuit.